The field of infrared spectroscopy has developed to provide devices for the prompt and efficient identification of chemical substances. Many of the Fourier-transform infrared (FTIR) spectrometer systems in commercial and laboratory use today make use of a Michelson interferometer to create a time varying light wave which is to passed through a sample of material. Variations in the light intensity due to interference are created by a moving mirror in the interferometer. Motion of the interferometer moving mirror is normally tracked by a positioning laser operating in parallel with the light source of the spectrometer, with the laser also entering the interferometer. The laser acts as a monochromatic light source so that the laser light intensity changes due to interference in the interferometer gives precise positioning information as to the change in position of the moving mirror.
The operation of such a Michelson interferometer FTIR spectrometer system is thus critically dependent on the position, speed, and control of the moving mirror. Since the motion of the mirror is desired to be constant, and since the mirror must stop and change direction, it is normal to detect the position of the mirror during its accelerating and constant velocity phases, so that precise information as to the location of the mirror can be utilized by the control circuitry of the FTIR. Such information is necessary to determine when to sample and digitize data. Exemplary systems for determining the start of scan and for determining the mirror position in an FTIR spectrometer are shown in U.S. Pat. Nos. 4,799,001 and 4,847,878 respectively, the disclosures of which are incorporated herein by reference.